1. Field of the Invention
This invention generally relates to antennas and, more particularly, to an antenna with periodic electromagnetic structures that suppress undesired propagation modes at the antenna""s resonant frequency.
2. Description of the Related Art
A planar antenna comprises a dielectric, a groundplane (counterpoise), and a radiator. Current is induced from the radiator to the groundplane through the adjacent dielectric, and radiates electromagnetic wave energy into free space. Because of its small size and flat shape, the planar antenna occupies a small space. The planar antenna can be mass-produced and is commercially viable because of its cost and size.
FIG. 17 shows a microstrip patch antenna, which is a widely used type of planar antenna (prior art). The microstrip patch antenna 100 is comprised of dielectric 102, a conductor or groundplane 104 located adjacent the dielectric 102, and a microstrip line 106 for feeding the current to a radiator or radiating element 108. The microstrip line 106 can be connected to a wireless communications device, receiver, transmitter, or transmit/receive switch (not shown). The electrical characteristics of the antenna 100 are affected in general by the dimensions of its elements and in particular by the size of the radiating element 108 and its distance from the ground plane 104, and the dielectric constant of dielectric 102. The design of slot, waveguide, flare-notch, dipole, monopole, and loop antennas involve similar design criteria, as is well known in the art.
Several issues must be considered in the design of an antenna to maximize throughput to a communication partner. The antenna can be shaped to maximally receive or transmit for various radiation patterns. Likewise, the antennas can be made directional. Antennas are designed to generate space waves at the resonant frequency in an intended mode. However, due to perturbations of the standing wave in the transmission media, irregularities in the dielectric material, improper matching between the antenna and the transmission media, or irregularities in the fabrication of the antenna, so-called xe2x80x9cleaky wavesxe2x80x9d are unintentionally generated that sap energy from the antenna operation in the intended radiation mode. Similar issues exist with the unintended creation of surface waves. Related problems involve the generation of evanescent waves, or non-propagated waves, when an antenna creates an unintended cutoff wavelength.
As is well understood in the art, an antenna acts to transform a guided wave in a transmission media, such as a coaxial cable or waveguide, into a space wave radiation mode propagated into a dielectric, such as air. When the phase velocity of the waves traveling in the transmission media is equal to the velocity of light (c), the transformation to the radiation mode space waves can be made without the loss of energy. Due to complex coupling between transverse electric (TE) and transverse magnetic (TM) modes in the transmission media, waves can be generated in the transmission media that have a phase velocity that is either greater than, or less than c. When the phase velocity exceeds c, leaky waves traveling in the transmission media are radiated, causing a continual energy loss. When the phase velocity is less than c, surface waves attenuate exponentially away from the transmission media surface, another loss of energy. It is an important to design antennas and antenna interfaces that mitigate the generation of leaky waves, surfaces waves, evanescent waves, and other forms of unintended radiation and wave propagation that rob power from the intended mode of radiation.
It would be advantageous if unintended modes of propagation associated with leaky waves, and the like, could be suppressed in an antenna.
It would be advantageous if a means existed of suppressing the unintended modes of propagation without changing the fundamentals of basic antenna design.
It would be advantageous if the unintended mode suppression means could be simply appended to conventional designs for patch, slot, waveguide, flare-notch, dipole, monopole, and loop antennas.
Wireless communications devices are expected to deliver high performance and great efficiency in a small package. Many wireless devices are expected to operate at a number of frequencies corresponding to the operation of the wireless device. Many wireless telephones can operate in the analog (AMPS), time division multiple access (TDMA), and code division multiple access (CDMA) modes. In addition, some wireless telephones incorporate global positioning system (GPS) receivers, and some incorporate local network transceivers for systems such as Bluetooth. It would improve the efficiency of wireless communications if antennas could be designed so that the unintended modes of propagation could be suppressed.
Accordingly, a family of antennas is provided with periodic electromagnetic structures to suppress unintended modes of propagation. Each antenna comprises a radiator resonant at a first frequency, with a proximate first dielectric and a counterpoise. The antenna conducts electromagnetic fields between the radiator and the counterpoise. A plurality of periodic electromagnetic structures propagate a radiating mode, while suppressing the propagation of a non-radiating mode. The periodic electromagnetic structures can be formed in the radiator, the counterpoise (when the counterpoise is distinctly distinguishable from the radiator), in the first dielectric, or in combinations of the above-mentioned elements.
The electromagnetic structures are a pattern of volumetric dielectric blocks having a predetermined shape and a predetermined spacing between blocks. For example, the shapes can be cylindrical blocks having predetermined diameters, cross-shaped blocks having predetermined arm widths and arm lengths, rectangular blocks having predetermined lengths and widths, or semi-spherical blocks having predetermined diameters. The blocks can be filled with a second dielectric with a second dielectric constant. Likewise, the volumetric blocks in either the radiator or counterpoise can be filled with the first dielectric material.
Using the above-described periodic electromagnetic structures, non-radiating modes can be suppressed in conventional patch, slot, waveguide, flare-notch, dipole, monopole, and loop antenna designs. Additional details of the above-mentioned antennas and a method for forming an antenna with periodic electromagnetic structures are described below.